dunnhumby's advice for FMCG advertisers on Facebook

"dunnhumby’s analysis shows that advertisers who planned campaigns with a relevant but broad reach, running for more than 6 weeks, at a frequency above 1 per person per week saw the greatest benefit from including Facebook on their media plans.
Combining these findings with those of the papers published last year, and applying the adjustment factors referenced, we are starting to see that for a campaign to achieve optimal sales lift, it should ideally be planned with a frequency ≥1 per person per week (not to be confused with f=1), across a duration of more than 6 weeks.
If an advertiser knows that people outside a core audience will not buy their products – perhaps due to them only being relevant to one life stage or one gender – then narrow targeting could be optimal, but more generally the rule is that a far greater return on ad spend will be achieved by aiming for as broad a relevant audience as possible."
Source:  Blog post by Facebook Business, 31st March 2017